Summary: The growth and mortality of the brown meagre, Sciaena umbra from the Gulf of Tunis, were investigated using a sample of 276 specimens, ranging from 15.3 to 49.2 cm total length and from 43 to 1565 g total weight. Specimens were collected from artisanal fisheries between October 2008 and September 2011. Otoliths were extracted, thin cross-sections were realized and radii were measured. The log-linear morphometric relationships between total length-total weight and total length-otolith radius were significant (p<0.05), showed positive allometry (b=3.15) and isometry (b=0.90), respectively. The marginal analysis suggested that only one growth increment was deposited per year. The maximum age of the brown meagre was 22 years for males and 31 years for females. The fit of the von Bertalanffy growth function was significantly different between sexes (p<0.05): TL∞=43.8 cm, k=0.145 y–1, t0=–4.88 y for males and TL∞=50.1 cm, k=0.105 y–1, t0=–5.71 y for females. The instantaneous total (Z) and natural (M) mortalities were estimated for each sex, resulting in an exploitation ratio (E=F/Z) of 0.43 for males and 0.46 for females.

The aim of the present study was to update the data relative to age and growth parameters of the brown meagre of the Gulf of Tunis by using thin cross-sections. Further, length-length (fish and otolith radii) and length-weight relationships, as well as mortalities (natural, M, and total, Z), were studied and the respective coefficients were estimated. Finally, an idea of the status of the investigated stock was derived by comparing the current exploitation ratio (E=F/Z) with the values conventionally related to a full exploitation. This parameter is indispensable for a rational management of fisheries, which is the only guarantee for a sustainable and responsible exploitation of a fishery resource.

The study was performed in the Gulf of Tunis (central Mediterranean Sea, 37°N 10°30'E). The brown meagre is caught by commercial fishing boats using artisanal fishing gears (mostly gillnets and trammel nets) and trawl, at depths ranging from 20 to 100 m, according to information provided by experienced fishermen who know the fishing area well. A total of 276 individuals were randomly collected on a monthly basis from October 2008 to September 2011. For each fish, the total length (TL) was measured to the nearest millimetre and the total weight (TW) to the nearest gram. The sex (male, female or undetermined) was determined by macroscopic observation of the gonads. Sagittal otoliths were removed, cleaned and stored dry in labelled envelopes for later treatment and examination.

Otoliths of S. umbra, like those of most Sciaenids species, are too thick to allow direct reading of the growth marks, so thin cross-sections through the core (nucleus) of the otoliths were used. The preparation method used to section into thin slices was described in Mahé et al. (2009)Mahé K., Bellail R., Dufour J.L. et al. 2009. Synthèse française des procédures d’estimation d’âge. IDM/RIC, IFREMER, Brest, France, 78 pp.. Otoliths were embedded in translucent polyester resin and two thin cross-sections (0.45 mm thick) were made using a high saw. These sections were then viewed under transmitted light against a white background, using a binocular microscope linked to a video camera connected to a computer. Paraffin oil was used to improve the visualization of growth increments. Each sample was then analysed using TNPC software (numeric treatment of calcified pieces, www.tnpc.fr).

The association of one opaque zone and one translucent zone was considered an annulus. The path of the growth marks was counted from the nucleus towards the tip of the inner face next to the sulcus (Fig. 1A). Measurements were taken from the otolith core to each opaque zone (Rn) and to the proximal margin of the otolith (Ro) to the nearest 0.01 mm.

where R is the number of times each fish is aged, Xij the i(th) ageing of the j(th) fish and X the mean age calculated for the j(th) fish. Only interpretations with an agreement between two readings were used in the successive elaborations.

Growth curve evaluation

The von Bertalanffy growth function is the most commonly used to describe fish growth. It was fitted to the observed length-at-age data as follow:

A total of two hundreds and 76 specimens composed of 121 males, 117 females and 38 specimens with undetermined sex were sampled. The sex-ratio of 1: 0.97 was not significantly different from the1: 1 ratio (χ2=0.7, n=238).

Males ranged in size from 17.6 to 43.3 cm and females from 19.1 to 49.2 cm. The mean TL of males (TL=28.98±0.52 cm) was not significantly different from that of females (TL=30.73±0.62 cm) (Mann-Whitney U test, n=238, p=0.06). Fish of undetermined sex were ranged between 15.3 and 26.3 cm (TL=20.04±0.50 cm) (Fig. 2). The total weight of fish was between 67 and 1063 g (TW=335.12±19.28 g) for males and between 84 and 1565 g (TW=424.01±27.73 g) for females. The mean TW was significantly different between the two sexes (Mann-Whitney U test, n=238, p=0.04). Total weight of fish with undetermined sex varied between 43 and 239 g (TW=99.83±8.29 g).

Fig. 2. – Frequency distribution of total length for males, females and individuals with undetermined sex of Sciaena umbra in the Gulf of Tunis.

The parameters of the length-weight linear regressions are provided for each sex and all individuals in Table 1. Our data suggested that S. umbra showed positive allometric growth. The sex effect was not significant on the length-weight relationships (ANCOVA, n=238, p>0.05).

Growth increments were counted on otolith cross-sections (Fig. 1A). Among the total sections examined (n=276), 252 (91.3%) yielded useful age estimates and 24 were discarded as they showed disagreement between the two readings. The value of APE (2.79%) demonstrated the very good reproducibility between readings, the reliability and the high precision of sectioned otolith for ageing the species.

The marginal increment analysis showed considerable individual variation (Fig. 3) but with a consistent pattern of higher values from August to February and lower values from March to June, the months corresponding to the mark formation period (Kruskall-Wallis test, p=0.009; SNK, p=0.001). The monthly trend in marginal increment analysis showed that only one mark is deposited per year. Significant correlation was found between mean marginal increment values and sea surface temperature (rs=0.12, p<0.05).

Fig. 3. – Monthly evolution of average marginal increment of Sciaena umbra in the Gulf of Tunis and the associated sea surface mean temperature (spawning period: May, June and July).

Age and growth

The relationships between TL and otolith radius (Ro) were calculated for males and females. Fish length and the radius of the calcified structure was closely correlated (p=0.00, Fig. 4). The analysis of covariance showed a significant difference in the length-radius relationship between the two sexes (ANCOVA, p<0.05).

Fig. 4. – Relation between otolith radius (Ro) and total length (TL) of Sciaena umbra in the Gulf of Tunis.

The observed age of fish ranged from 1 to 31 years (Fig. 1B). Females reached a higher maximum age (31 years) than males (22 years). No significant differences in mean ages between males (3.74±0.39 years) and females (4.27±0.43 years) were observed (Mann-Whitney U test, n=226, p=0.34). Most of the fish, accounting for 69% of the total sample, were between 1 and 3 years old (Table 2). Fish older than 9 years old were poorly represented in the sample. More than 50% of the maximum length was achieved by the first year.

The growth curves of age-at-observed length data were well described through von Bertalanffy fits (Fig. 5). Significant differences were found between the von Bertalanffy growth curves of both sexes (Hotelling’s T2 test=10.07, p<0.05).

Fig. 5. – von Bertalanffy growth curves for males and females of Sciaena umbra in the Gulf of Tunis.

The von Bertalanffy relationships between age and length, derived from the assumed annual periodicity of the growth increments, were described by the growth parameters: TL∞=43.83 cm, k=0.145 year–1, t0=–4.88 years for males and TL∞=50.10 cm, k=0.105 year–1, t0=–5.71 years for females.

Mortality and exploitation rate estimates

Total mortality, corresponding to the slope of the descending limb of the catch curves, was Z=0.51 year–1 for males and Z=0.41 year–1 for females (Fig. 6). Values of natural mortality were M=0.29 year-1 for males and M=0.22 year–1 for females. Calculation of fishing mortality gave F=0.22 year–1 for males and F=0.19 year–1 for females. With the values of M and F known, the exploitation ratios were computed as E=0.43 for males and E=0.46 for females.

Fig. 7. – Auximetric grid showing the relationship performance of Sciaena umbra from different areas. The different studies are indicated with different colour. Growth performance of males are represented by diamonds, females by circles and combined sex by triangles.

These variations in growth performance between different regions are probably the result of the following factors:

The results obtained from the current study provide an actualization of practical biologically related parameters for stock assessment and management of the brown meagre of the Gulf of Tunis. Regular follow-up of life history traits of this species are necessary for its optimal management on the Tunisian coasts.